This invention relates to a multi-layer film stack suitable for use as an anti-reflective coating on a transparent substrate such as glass. More particularly, this invention relates to a multi-layer film stack which is applied onto a glass substrate to impart anti-reflective properties to a coated glass article which is normally viewed at a non-normal angle, such as a vehicle windshield.
Coatings on glass are commonly utilized to provide specific energy attenuation and light transmittance properties. Additionally, coatings are designed to reduce reflections from interfaces between individual coating layers and the glass when a plurality of coatings are applied onto a glass substrate. The coated articles are often utilized singularly, or in combination with other coated articles, to form a glazing.
The attributes of the resulting coated glass substrate are dependent upon the specific coatings applied to the glass substrate. The coating compositions and thicknesses impart energy absorption and light transmittance properties within the coated article while also affecting the spectral properties. Desired attributes may be obtainable by adjusting the compositions or thicknesses of the coating layer or layers.
Anti-reflective coatings on glass are utilized to reduce the surface reflection of optical components and to reduce the reflectance of an interface between optical media with different refractive indices. The reduction of visible reflection is achieved by the principle of optical interference. When light impinges on the air-film, film-film, and film-glass interfaces, a portion of the beam is reflected at each interface. By proper choice of thin film materials and thicknesses, the individual reflected light beams can destructively interfere, thereby reducing the observed visual reflectance. However, adjustments to enhance a specific property can adversely impact other transmittance or spectral properties of the coated glass article. Obtaining desired spectral properties is often difficult when trying to combine specific energy absorption and light transmittance properties in a coated glass article.
Conventional multi-layer anti-reflective films generally follow a standard design formula to optimize the reduction of visible light reflected from the interfaces of the coated glass substrate. The standard design parameters suggest the use of a two layer coating, of both high and low indices, with each coating having a thickness determined by l/(4*n), where l is a design wavelength in the visible region, and n is the refractive index of the coating. These design parameters provide an anti-reflective film stack which minimizes visible reflection from the coated glass article at a normal angle which is normal to the glass article.
It would be desirable to manufacture an anti-reflective transparent substrate that is viewed, for the most part, at a non-normal angle.